Can't find old topic on clock compensation

Projects Programming
17

Just for interest, I wrote this fairly simple sketch:

void setup() { }

void loop() 
  {
  OSCCAL = analogRead (A0) >> 2;
  }

That takes the analog input from A0, divides it by 4 to get it into the range 0 to 255, and then puts that in the oscillator calibration byte.

I set the low fuse to: 0xE2 (on an Atmega328P). This uses the internal oscillator, and outputs the clock on pin D8 (pin 14 on the chip). I connected my scope to that pin.

Cranking a pot around from one end of the scale to the other I got:

Lowest: 4.8 MHz
Highest: 15.7 MHz

The datasheet warns:

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.

However certainly you should be able to empirically determine the value to correctly run at 8 MHz, and save that somewhere (eg. EEPROM).